Conventional methods of down converting a Radio Frequency (RF) signal to baseband require two conversion steps. The RF signal is first down converted to an intermediate frequency (IF) signal. Then, the IF signal is down converted to a baseband signal. In a mobile telecommunication environment, this requires a radio frequency receiver (RFR) chip, an intermediate frequency receiver (IFR) chip, a baseband receiver chip, and other associated surrounding chips, all of which are expensive for mobile phone manufacturers.
A direct conversion enables the direct conversion of RF signals to baseband signals in a single step. Thus, direct conversion eliminates the need for the RF to IF conversion step, and thus, the IFR chip.
One of the problems associated with direct conversion is that it results in very high direct current (DC) offset levels. These unwanted DC offsets include static DC levels as well as time varying DC levels. The sources of static and time-varying DC offsets include circuit mismatch, LO self-mixing, and interferer self-mixing, each of which may vary with gain setting, frequency, fading, and temperature. If such DC offsets are not cancelled, they degrade signal quality, limit dynamic range through saturation, and increase power consumption.
What is needed is a circuit and method that cancels DC offsets for direct conversion architectures. What is also needed is a circuit and method that acquires and cancels DC offsets in a fast and efficient manner for direct conversion architectures.
There are two main methods for DC offset cancellation in nowadays. One simple and straightforward method to cancel DC offsets is ac-coupling, as shown in FIG. 1. The circuit for DC offset cancellation 10 includes at least an amplifier or a filter 11, wherein input of each amplifier or filter 11 is coupled with an ac capacitor 13 for isolating the DC offset. This method is suitable and cost effective where the signal spectrum does not have so much energy at dc and the ac-coupling does not degrade system performance. However, large off-chip capacitors are required by using this method, and response time is slow.
Another method to cancel DC offsets utilizes a low-pass filter 27 via negative feedback, as shown in FIG. 2. The circuit for DC offset cancellation 20 includes at least an amplifier or a filter 21, wherein each amplifier or filter 21 is coupled with a DC offset cancellation loop which is a negative feedback loop. The DC offset cancellation loop as shown in FIG. 2 includes the low-pass filter 27 to detect the DC offset therein output of each amplifier or filter 21, and cancel the DC offset therein input of each amplifier or filter 21. While using this method in a system having multistage amplifier and filter, each stage is needed to couple with a DC offset cancellation loop including a low-pass filter for canceling the DC offset therein input of multistage amplifier and filter. Therefore, the implementation of low-pass filters for canceling the DC offset causes the larger area and higher power consumption.